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Nickel crystallites

The activity loss measured here is caused by recrystallizations. This was demonstrated by using scanning electron microscopy to determine nickel crystallite size in the same catalyst samples. These tests revealed that the catalyst used in demonstration plants has only a slight tendency to recrystallize or sinter after steam formation and loss of starting activity. [Pg.131]

It was observed that per unit sur ce area, small nickel crystallites were more reactive toward methane than were large crystallites. [Pg.458]

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. [Pg.14]

It is particularly helpful that we can take the Cu-Ni system as an example of the use of successive deposition for preparing alloy films where a miscibility gap exists, and one component can diffuse readily, because this alloy system is also historically important in discussing catalysis by metals. The rate of migration of the copper atoms is much higher than that of the nickel atoms (there is a pronounced Kirkendall effect) and, with polycrystalline specimens, surface diffusion of copper over the nickel crystallites requires a lower activation energy than diffusion into the bulk of the crystallites. Hence, the following model was proposed for the location of the phases in Cu-Ni films (S3), prepared by annealing successively deposited layers at 200°C in vacuum, which was consistent with the experimental data on the work function. [Pg.122]

It was assumed that the nickel crystallites are rapidly enveloped in a skin of a copper-rich alloy, from which diffusion towards the center of each crystallite then takes place. If xx and x2 are the atomic fractions of copper in the two equilibrium phases and x is the atomic fraction of copper in the alloy film under consideration, then the crystallites in the annealed film may have a variety of forms. Solid solutions occur at either end of the composition range but the values of Xi and x2 at 200°C are <0.1 and 0.8. Hence, over much of the composition range (i.e., where x lies between X and xi), the Cu-Ni films should consist of crystallites with a kernel which is almost pure nickel (composition xi) enveloped in a skin of a copper-rich alloy (composition x2). Eventually, when x is only slightly larger than Xi, the alloy skin does not completely surround the nickel crystallites small patches of alloy (x2) and almost pure nickel ( ci) are both exposed. [Pg.123]

Skeletal nickel consists of highly-dispersed nickel with a large surface area [68, 91-96], the structure often being likened to a sponge [51,74], The activity of the catalyst is proportional to the surface area and hence the degree of nickel crystallite dispersion [26,76,91], The nickel crystallites are about 1-20 nm in size [24,92,94-96], and decrease in size with decreasing temperature... [Pg.147]

As discussed, XRD has for many years been the standard, everyday characterization method for solid catalysts, and in almost every laboratory in this field there is access to an X-ray diffractometer. This instrument allows a wide variety of different characterizations, but there are also limitations of such equipment. For example, the limited resolution of an in-house diffractometer may often be insufficient for a detailed analysis. This point is illustrated in Fig. 5a, which shows the diffractogram of an industrial type steam-reforming catalyst consisting of nickel crystallites on a spinel support (35). The Ni(lll) and the spinel(400) lines overlap so that a detailed analysis is impossible. This problem can be overcome if the XRD... [Pg.324]

The active sites for the oxygen adsorption, which are found on the surface of NiO (250) but not of NiO (200), are to be identified with anionic vacancies because this high heat of adsorption is not caused by the sorption of oxygen on the nickel phase (13). The decrease in the capacity for adsorption of oxygen at 30°C. when the temperature of oxide preparation is increased from 200° to 250°C. is explained by the reduction of surface nickel ions, sites for the adsorption only of oxygen, and the formation of nickel crystallites whose surface atoms may be active towards the adsorption of oxygen at 30°C. Recession of nickel ions below the surface for NiO (250) may also contribute to this decrease. [Pg.296]

Table 1 gives the average sizes of nickel crystallites measured by X-ray line broadening analysis on (111) reflections, before and after the five hydrogenation runs. They increase moderately and even decrease for RNiFe. This confirms that the BET area loss could be due in part to a poisoning which reduces the capacity of nitrogen adsorption. However, measurements of the metallic surface area should also be done to confirm possible surface poisoning. [Pg.233]

Range 2a xx < x < (xj + Ax). Small patches of alloy with x = x2 cover crystallites of almost pure nickel (x = x,). The alloy skin does not completely surround the nickel crystallites. [Pg.77]

Transmission electron micrographs and XPS results have been used to show that a catalyst, with a high silica content in the matrix, prevents nickel dispersion (16,18). In fact, in a FCC with a Si-rich (Si/Al = 4.3) surface, XPS data has indicated that calcination and steaming cause nickel (and vanadium) migration to the cracking catalyst surface where nickel sinters. As a result, nickel crystallites 50... [Pg.354]

Avoiding carbon deposition on the catalyst is a major challenge [2, 3]. Carbon can be present as graphite-like coke and in the form of whiskers, or carbon nanofibers. The latter lead to detachment of the nickel crystallites from the support and breaking of the catalyst pellets. This may cause blockage of the reformer reactor tubes and the formation of hot spots. Higher hydrocarbons exhibit a larger tendency to form... [Pg.443]

The experimental results have been used as a basis for building kinetics models 110-113). Carbon formation kinetics has also been included in the microkinetics models. The models assume that the carbon filaments are formed by carbon atoms diffusing through bulk nickel crystallites. Recent investigations have also indicated that surface diffusion processes can be more important than was believed in the filament formation mechanism 114). When the irreducible heat transfer limitation was taken into account, providing an improved estimate of the real catalyst surface temperature, the model was able to predict both our own kinetics data 110 113) as well as the intrinsic kinetics reported by Xu and Froment 115) for the reaction in the presence of a similar catalyst (nickel on Mg-Al203 spinel). [Pg.378]

In the account of this chemisorption work, hydrogen chemisorption was also described. Hydrogen chemisorption was consistently less than the CO (after correction for substrate contribution) and it is suggested that this difference is owing to the need for nickel crystallites large enough to provide adjacent sites for dissociative adsorption of hydrogen atoms, whereas the CO can adsorb as an undissociated molecule on individual surface nickel atoms. [Pg.433]

The morphology of the carbonaceous deposits generated by the decomposition of propane on the nickel and nickel-potassium catalysts was studied by TEH. It was found that the reaction of propane, at elevated temperatures (375 0-500 0, produced filamentary carbon on the catalysts. These filaments were hollow, 100A-900A in diameter and had highly orientated nickel crystallites at the top,... [Pg.181]


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See also in sourсe #XX -- [ Pg.180 ]




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Nickel crystallite size

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